At some point prior to the 23rd century, the nebula contained several inhabited planets. By the 2260s, the ancient civilizations that had lived on these planets had long died out in unknown circumstances.
While investigating the death of colonists on several Federation planets, including Ingraham B, in 2267, Commander Spock hypothesized that these civilizations had fallen in the same way. However, archaeological evidence was too sketchy to support his hypothesis. (TOS - Star Trek 2 novelization: Operation -- Annihilate!)